Dibenzoylmethane compounds are known to absorb ultraviolet radiation in the 320 to 400 nm range (UV-A) and accordingly act as UV-A light screening agents. However, no UV-A blocking sunscreen can be used alone if absorbtion in a wide range of UV-radiation is required. Thus, UV-A filters usually must be combined with UV-B absorbing agents. Unfortunaltelly, when dibenzoylmethane compounds are used alone or in combination with UV-B screening agents, the dibenzoylmethane type UV-A screening agents are photolabile and it is necessary to photostabilize the dibenzoylmethane type UV-A filters.
Photostabilization in this context means to maintain a constant or nearly constant protection of human skin or hair by such a UV-A screening agent against ultraviolet light in the range of 320 to 400 nm.
Up to now, UV-A screening agents have been photostabilized by adding specific UV-B filter compounds known for this purpose. For example the UV-A screening agent 4-tert.butyl-4'-methoxydibenzoylmethane (U.S. Pat. No. 4,387,089) sold as PARSOL 1789.RTM. by F. Hoffmann-La Roche AG, is stabilized by Octocrylene (see EP-A-780 119), benzylidenes (see EP-A-754445), especially by methylbenzylidene camphor, or by a polymer of the benzylidene malonate silicone type (see EP-A-708080), which photostabilizing compounds are all UV-B filters.
The German patent publication DE 19540952 or the corresponding International publication WO 9717054 address to the problem of unsufficient photochemical stability of dibenzoylmethane type UV-A filters. It is suggested to use dicyanodiphenylethylene derivatives as UV-A filters instead of dibenzoylmethane type UV-A filters in cosmetics also containing compounds absorbing UV-B. The dicyanodiphenylethylene derivatives absorb in the 320-380 nm region and are stable to light. There is no teaching nor any guidance in the German patent publication DE 19540952 that would prompt the skilled person to use the well established photolabile dibenzoylmethane UV-A filter and to photostabilize it by adding dicyanodiphenylethylene derivatives.